1. Field of the Invention
The invention relates to an apparatus for drive control, power conversion, and start-up control in a PMBDCM or a two-phase SRM drive system.
2. Description of Related Art
Variable speed motor drives are expected to play an increasingly important role in improving home appliances particularly in their ability to meet existing and proposed federal efficiency requirements. In such motor drives, cost reduction is important. Cost reductions can come from one or more subsystems, i.e., the motor, power converter, and controller. Of all existing motor drive systems, switched reluctance machines (SRMs) offer the greatest potential for cost reduction in their subsystems, and the power converter is the primary subsystem where cost can be substantially reduced. Following is a brief description of related art power converter topologies for a two-phase SRM.
FIG. 1 illustrates a related art asymmetric power converter for driving a two-phase SRM. Power converter 100 has two controllable and two uncontrollable power devices for each phase winding 101, 102 of the SRM. Therefore, four controllable 103-106 and four uncontrollable 107-110 power devices are required for power converter 100 to operate. The primary advantage of power converter 100 is that it gives full controllability in terms of its ability to apply full positive or negative direct current (do) link voltage and, therefore, does not diminish or restrict any operating mode of the SRM. The disadvantage of this power converter topology is that it uses eight power devices. A more detailed description of power converter 100's circuit operation may be found in “Switched Reluctance Motor Drives”, R. Krishnan, CRC Press, June 2001.
FIG. 2 illustrates a related art single switch-per-phase power converter for driving a two-phase SRM. Power converter 200's circuit topology is based on splitting a dc input source voltage 201 equally to the machine side power converter. This results in a circuit requiring one controllable and one uncontrollable power device per phase winding 202, 203. Therefore, overall, power converter 200 requires two controllable power devices 204, 205 and two uncontrollable power devices 206, 207 for a two-phase SRM. The major advantage of this circuit design is that it uses a reduced number of power devices (e.g., a total of four) compared to the asymmetric converter. The disadvantage of this circuit is that it reduces the available dc source voltage by half and, therefore, doubles the current rating required for the devices and for the machine, resulting in low efficiency machine operation. A fuller description of this circuit may also be found in “Switched Reluctance Motor Drives”, R. Krishnan, CRC Press, June 2001.
FIG. 3 illustrates a related art C-Dump power converter for driving a two-phase SRM. Power converter 300's circuit uses three controllable power devices 301-303 and three uncontrollable diodes 304-306, resulting in the use of six power devices. This is an intermediate circuit between those illustrated in FIGS. 1 and 2. The operating modes are somewhat restricted for this circuit, since it can apply full dc source voltage 309 to machine windings 307, 308 only in the positive direction. Furthermore, this circuit requires an external inductor 310 or a resistor (not shown) to dissipate the energy stored in C-dump capacitor 311. Use of external inductor 310 increases the cost, whereas the use of the power resistor (not shown) will result in a lower efficiency of the system and higher package volume, due to increased thermal considerations. Therefore, this circuit is not ideal for use with two-phase SRMs. A more detailed description of this circuit may be found in “Switched Reluctance Motor Drives”, R. Krishnan, CRC Press, June 2001 and in Miller et al., U.S. Pat. No. 4,684,867, published in Aug. 4, 1987.
FIG. 4 illustrates a related art single switch-per-phase power converter for driving a two-phase SRM. Power converter 400 requires one uncontrolled 401, 402 and one controlled power device 403, 404 per phase 405, 406, and therefore, requires four power devices to function. Furthermore, power converter 400 requires a special winding in the machine, known as a bifilar winding. This special winding increases the copper volume in the machine windings, resulting in increased cost for the machine. Additionally, power switches 403, 404 experience higher voltage stresses due to the leakage inductance between the windings of the respective phase. This leakage inductance can be minimized but cannot be eliminated in a practical machine. Therefore, this converter circuit is not widely used, despite the fact that a full do source voltage 407 can be impressed on the machine with full controllability of the current. A more in depth description of this circuit may be found in “Switched Reluctance Motor Drives”, R. Krishnan, CRC Press, June 2001 and in Miller, U.S. Pat. No. 4,500,824, published Feb. 19, 1985.
All other power converter circuit topologies fall into one of the above-described categories, in terms of the total number of power devices required for their operation. From the foregoing, it may be seen that a minimum of four power devices are required for operating a related art two-phase SRM.
Generally speaking though, commercial power converters used to drive a two-phase SRM usually have more than two controllable switches and more than two diodes. Circuits requiring only two controllable switches and two diodes have the disadvantages of high power loss, low efficiency, and sometimes a bifilar winding in the machine, thereby reducing the power density of the machine. Therefore, existing solutions are not attractive with regard to considerations of high efficiency operation, full range of speed control, compactness in the converter's packaging and, most importantly of all, the overall cost of the system.
The fundamental challenge in power converter development has been to reduce the number of power devices, both controllable and uncontrollable, to a level corresponding to that of a single-quadrant chopper drive, such as is commonly used in a dc motor drive or in a universal motor drive. A description of these drives is provided in “Switched Reluctance Motor Drives”, R. Krishnan, CRC Press, June 2001. When the number of power devices has been reduced to this level, a brushless SRM drive becomes commercially competitive for variable speed applications. Moreover, the brushless SRM has the superior advantage of high efficiency, since there are no brushes and commutators in the SRM. Also, the brushless SRM is further endowed with high-speed operability, high reliability, maintenance-free operation, greater overload capability and, most of all, a cost advantage over the dc motor drive.
All reference material cited herein is hereby incorporated into this disclosure by reference.